How many 80lb bags of Quickrete will it take for a 20’ x 12’ x4” slab?
Use their calculator!
Enter the area (12x20 is 240 sqft) and the depth of your slab (4in). It will break it down into the required number of bags.
There are some good alternatives to mixing your own bags. See answers under the earlier question Best way to source concrete for a long fence project
For a project this size (I had a similar one) I was wisely advised to get a truck, pump and finishing crew. I have completed smaller sizes alone, with bags and a mixer.
Also note that if you are mixing this yourself, the time spread and mixing variations alone, for a large slab, might give you batch inconsistencies that can become an eyesore. But this is a matter of personal skills & priorities of course.